Method and electric furnace for melting vitreous materials

ABSTRACT

An electric furnace ( 1 ) for melting vitrifiable materials (V), in particular for the manufacture of vitreous mosaic materials and ceramic frits as well as for the vitrification of waste, where the primary material is frequently changed, comprises a melting tank ( 2 ) for containing a molten bath ( 3 ) with a head (B), having a floor ( 4 ) and side walls ( 5 ), channels ( 6 ) for discharging the molten materials, a crown ( 13 ) which is situated above the floor ( 4 ), means for introducing a primary batch of vitrfiable materials (V) into the tank ( 2 ) and for depositing a covering layer (C) on the molten bath ( 3 ), and a plurality of electrodes ( 9 ) with a predetermined shape and position, situated inside the tank ( 2 ) for melting completely the vitrifiable materials (V) by means of diffused electric currents. The electrodes ( 9 ) substantially rest on the floor ( 4 ) so as to reduce to a minimum the head (B) of the molten bath ( 3 ), with a consequent reduction in the time required to change the primary batch and the power consumption.

TECHNICAL FIELD

The present invention relates to the technical field of vitreousmaterials and in particular relates to a method and an electric furnacefor the production of vitreous mosaic materials, ceramic frits andsimilar products as well as for the vitrification of waste.

BACKGROUND ART

It is known that batch furnaces or crucible furnaces or continuous canalfurnaces, which differ from each other as regards the procedures for themelting process, may be used for the production of vitreous materials,such as, for example, a mosaic product composed of a vitreous paste.

In batch furnaces, the raw materials contained in the crucible arefirstly heated to a high temperature in order to melt them and form thevitreous mixture, and other raw materials such as, for example, silicasand, are then added in order to obtain an opacifying effect and acrystalline grain; finally, said materials are cooled, before beingconveyed to suitable forming machines in order to produce the endproduct, for example a vitreous mosaic material.

Owing to these process characteristics, crucible furnaces are suitablefor small production outputs, ranging between 100 and 3000 kg ofvitreous product per day. In continuous furnaces, the various stages ofproduction are distributed spatially, but are performed simultaneously.The raw materials forming the primary vitrifiable mixture are meltedcontinuously inside a tank which is connected by means of submergedpassage or gully to a canal. Substances necessary for producing theopacifying effect are added into the canal. A casting tank whichsupplies the forming machines is situated at the other end of the canal.

Unlike crucible furnaces, continuous furnaces are suitable for greaterproduction outputs, exceeding 5000 kg of vitreous product per day.

Ceramic frits are produced industrially in furnaces of the continuoustype. At present melting furnaces of the oxygen-combustion type are inparticular preferred. In view of their small dimensions, for theseapplications the use of efficient, but costly systems for pre-heatingthe comburent air, such as regenerators, as used in large glass tankfurnaces, is avoided. Thus, the fumes are conveyed directly to the flue,still at a high temperature. Owing to the notable environmental impactof the flue emissions, in accordance with recent legislation, furnacesfor ceramic frits must also be equipped with a fume filtration device ofthe sleeve filter type. These plants not only have a high installationcost, but are also costly to manage on account of the large volume offumes due also to mixing with the ambient air necessary in order tolower the temperature to levels compatible with the filters used.

Furnaces for vitrification of waste at present constitute a type ofplant which is still in the experimental stage. The raw materials whichform the vitrifiable mixture consist, wholly or partly, of toxic wasteof inorganic origin, such as for example the residual matter from RSUincinerators and the dross resulting from the processing of metals andcomposite materials containing asbestos. The aim of this type oftreatment of dangerous waste is to produce glass which has a suitablechemical stability and which, even though not completely refined, may bereused as a semi-processed product in the ceramics, glass fibers andfoamed glass industry for thermal insulation or files to be used in thebuilding sector.

In all the abovementioned production processes, the method of meltingthe vitreous products, such as that for ceramics frits and forvitrification of waste, is characterized by the production of glasswhich is not entirely devoid of internal air bells, namely is notrefined. The vitrifiable mixture, moreover, may contain elements whichevaporate easily and may therefore have a significant and problematicimpact on the environment.

Finally, since the composition of the mixture is subject to frequentchanges in order to produce products with a different color andopacifying effect, in order to speed up the material replacementoperations, it is preferred to use very low heads of glass.

Generally a drawback of certain solutions consists in the fact that thethickness of the layer of vitrifiable mixture, which is deposited on thesurface of the molten bath, is limited and is not sufficient to screenthe dispersions which are irradiated towards the crown of the furnace.Thus, some components in the mixture may easily evaporate and minglewith the discharge fumes, thereby contaminating them.

Owing to their high temperature and harmful content, the existinglegislation governing pollution requires the use of costly filtrationplants.

German patent No. 1,080,740 discloses a furnace for vitreous materialshaving a tank with a polygonal shape in plan view, suitably designed toensure a uniform temperature inside the molten bath. Electrodes aremounted on the side walls of the furnace and towards the central zone ofthe tank and, being suitably energized by electric transformers generatea diffused current within the molten bath. This diffused current heatsthe vitreous mixture contained in the tank as a result of the Jouleeffect. During continuous operation, the vitrifiable mixture isdeposited on the upper surface of the molten bath so as to form auniform layer, while an opening on the floor and close to the corner ofthe tank allows the molten glass to flow out.

A disadvantage of the solution considered consists in the considerablethickness of the head of glass, due to the shape of the tank and thearrangement of the electrodes. This constitutes a limitation when thevitrification mixture must be changed frequently, since it increases thetime required for changing the mixture of raw materials to be vitrified.

A second disadvantage of the solution in question consists in the factthat the ends of the electrodes are freely immersed in the molten bath,resulting in a high intensity of current in the vicinity of the saidends. For this reason, the immersed ends of the electrodes are subjectto rapid wear.

DE-C-564491 discloses an electric furnace with a plurality of electrodesplaced on the floor. Each electrode has a variable cross-section and aninterruption in correspondence of a central area of the bath. Thisinterruption defines an internal space in which the convection currentsof the melting bath originate. The variable cross-section of theelectrodes is specifically directed to provide a vertical extension ofthe melting bath and does not prevent an increase in the overall head,change time and power consumption of the melting process.

U.S. Pat. No. 4,143,232, which is considered the nearest prior art onwhich is based the preamble of claims 1 and 5, comprises a glass furnacecomprising three groups of electrodes which are positioned in themelting tank at three different levels in order to yield convectioncurrents in the molten bath. The groups of electrodes which are placedat the upper level far from the floor play a very important role duringthe process as they allow to keep the molten bath flatter and morestable. Moreover, the other two groups of electrodes are placed atdifferent levels and distances from the side walls of the tank in orderto improve the control of the convection currents and of the shape ofthe melting bath. This known furnace has no provision for reducing to aminimum the head of the molten bath, the time for changing the primarybatch and the power consumption.

DISCLOSURE OF THE INVENTION

A main object of the present invention is that of eliminating thedrawbacks mentioned above by providing a method and a furnace for theproduction of vitreous mosaic materials, ceramic frits and similarproducts as well as for the vitrification of waste, which have thecharacteristics of low-cost and limited impact on the environment.

A particular object is that of providing a cold-crown furnace which isable to lower the temperature and the quantity of polluting substancescontained in the fumes discharged into the atmosphere.

A further object of the invention is that of providing an electricfurnace which allows a reduction in the time required to change thevitrifiable material.

Another particular object is that of providing an electric furnace whichis configured so as to limit the specific power consumption.

These objects, together with others which will appear more clearlybelow, are achieved by a method for melting vitrifiable materials, inparticular for the production of vitreous mosaic materials and ceramicfrits as well as for the is vitrification of waste, in accordance withclaim 1.

As a result of this method, it will be possible to reduce the timerequired for changing the primary batch and the power consumption.

According to a further aspect, the invention provides an electricfurnace for melting vitrifiable materials, in particular for theproduction of vitreous mosaic materials and ceramic frits as well as forthe vitrification of waste, where the primary material is frequentlychanged, in accordance with claim 5.

Preferably, the electrodes are substantially cylindrical and straightand have a length at least equal to the distance between the oppositeside walls of the tank and are arranged substantially parallel to eachother at a given mutual distance so as to optimize the distribution ofthe electric current inside the molten bath.

Owing to this characteristic feature it is possible to obtain ahomogeneous distribution of the power within the molten bath.

Conveniently the electrodes have one longitudinal end rigidly secured toa side wall of the tank and the other longitudinal end in contact withthe opposite side wall so as to be slightly compressed or tensioned atthe tip. As a result of this measure it is possible to ensure theelectrical continuity even after possible breakage or cracking of theelectrodes. Moreover, the characteristic high degree of wear of the tipsis avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will be more clearlyunderstood in the light of the detailed description of some preferred,but not exclusive embodiments of the electric furnace according to theinvention, illustrated by way of a non-limiting example with the aid ofthe accompanying plates of drawings in which:

FIG. 1 shows a sectioned side view of the furnace as a whole;

FIG. 2 shows a cross-section through the molten bath and the electrodes;

FIG. 3 shows a plan view of a preferred example of embodiment of thefurnace according to the invention;

FIG. 4 shows a plan view of a second preferred example of embodiment ofthe furnace according to the invention;

FIG. 5 shows graphs of the specific electrical consumption parameterizedaccording to the value of the average daily gather.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With particular reference to the said figures, the description belowrelates to an electric furnace for melting vitrifiable materials, inparticular for the manufacture of vitreous mosaic materials and ceramicfrits as well as for the vitrification of waste according to theinvention, said furnace being denoted in its entirety by the referencenumber 1.

The furnace 1 comprises a melting tank 2 for containing a molten bath 3,which is essentially formed by a floor 4 and by side walls 5, which areoften referred to as “palisades”. Suitable discharge channels 6 areformed in the floor 4 in order to allow and facilitate the removal ofthe molten materials from the tank 2.

Movement and transporting means 7 are envisaged for introducing into thetank 2 a primary batch of vitrifiable materials V and for depositing acovering layer C on the molten bath 3. The movement and transportingmeans 7 may consist of a conveyor belt 8 or similar devices which passthrough the mouth of the furnace, not shown in the drawings.

During start-up of the furnace, conventional heating means, preferablyof the combustion type (not shown in the drawings and of a type knownper se) are used for melting, at least partially, the vitrifiablematerials V and for forming, in this way, the molten bath 3. After themolten bath 3 has been created, it is possible to commence heating ofthe furnace using electric means.

Conveniently, electric heating of the furnace is performed by means ofan electric current diffused in the molten bath 3, which currentgenerates heat as a result of the Joule effect. For this purpose, aplurality of electrodes 9, which have a predetermined shape andposition, are provided inside the tank 2 in such a way that the electriccurrent circulates between them.

The electrodes 9 may be supplied with a single-phase alternating currentR-S, generally by connecting half of the electrodes 9 to the conductor Rand the remaining half to the conductor S. In another example ofembodiment, the electrodes may be supplied with a three-phasealternating current R-S-T.

According to the invention, the electrodes 9 substantially rest on thefloor 4 so as to reduce to a minimum the head B of the molten bath 3,with a consequent reduction in the time required to change the primarybatch and the power consumption.

Preferably, the electrodes 9 are cylindrical and straight and arearranged substantially parallel to each other at a given mutual distanceD,D′ so as to optimize the distribution of the electric current insidethe molten bath 3.

The length L of the electrodes 9 is at least equal to the distancebetween the opposite side walls of the tank 2. In this way the surfacearea of the electrodes 9 in contact with the materials of the moltenbath 3 is increased. Moreover, the electrodes 9 have one longitudinalend rigidly secured to a side wall 5 of the tank and the otherlongitudinal end in contact with the opposite side wall 5 so as to beslightly compressed or tensioned at the tip. As a result of the amplearea of contact between the electrodes 9 and the molten bath 3 and theabsence of electrodes having their tips freely immersed in the moltenbath 3, it is possible to limit the current intensity and consequentlythe wear phenomena.

The stresses associated with unforeseeable thermal settling movementscould give rise to breakage fissures or cracks. The slight compressionto which the electrodes 9 are subject helps ensure the electricalcontinuity also in the case of breakage or cracking of the electrodes 9.

The side walls 5 of the tank 2 have a minimum height H greater than themaximum value of the head B of the molten bath 3, plus the maximumthickness S of the covering layer C. This minimum height H of the sidewalls 5 of the tank 2 may be between 35 to 60 cm if the diameter of theelectrodes 9 is between 1″ and 2½″. If, on the other hand, the diameterof the electrodes 9 is between 1½″ and 2″, then the minimum height H ofthe side walls 5 is preferably between 40 and 60 cm.

Since the diameter of the electrodes 9 is comparable with the head B ofthe molten bath 3, the electrodes could hinder discharging of the moltenglass. For this reason, the discharge channels 6 extend at leastpartially underneath the electrodes 9.

The discharge channels 6 may comprise at least one main receiving canal10 connected to the outside of the furnace by means of a discharge gully11. The main canal 10 may have a direction substantially parallel to theelectrodes 9.

In another embodiment, the main canal 10 may have a directionsubstantially perpendicular to the electrodes 9. Moreover, it ispossible to use also a plurality of secondary receiving canals 12connected to the main canal 10, particularly in the configuration wherethe electrodes 9 are perpendicular to the main canal 10.

Conveniently, the main canal and secondary canals 10, 12 are transverseto each other and extend completely underneath the electrodes 9.

The furnace is closed at the top by a crown 13 which is situated abovethe floor 4 and the side walls 5.

Operationally speaking, a primary batch of vitrifiable materials V isintroduced into the tank 2 via the inlet mouth of the furnace (not shownin the drawings) and by means of movement and transporting means 7.

Solely during the initial cold furnace stage, the charge of materials Vis pre-heated using conventional heating means so as to melt them atleast partly and form the molten bath 3 with a head B. At this point,heating of the furnace is started by energizing the electrodes 9 withsingle-phase or three-phase electric current so as to melt completelythe vitrifiable materials V.

A covering layer C of vitrifiable materials in the solid state isdeposited on the upper surface of the molten bath 3 so as to contain theheat dispersions of the bath and screen the crown 13 of the furnace.

Owing to the position of the electrodes 9 resting over the whole oftheir length on the floor 4, a reduction in the head B of the moltenbath 3 is obtained, with a consequent reduction in the time required forchanging the primary batch and the power consumption.

FIG. 5 shows some graphs which illustrate the specific energyconsumption, normalized with respect to the unit of mass of glassproduced, in the case of a furnace with a floor having a square shapeaccording to the invention, and parameterized for the specific gather[ton/(day m² _(floor)].

The specific consumption is dependent upon both the dimensions of thefurnace and the quantity of glass produced, expressed as tons of glassoutput per day. With an increase in the dimensions there is obviously anincrease in both the dispersions and the quantity of glass which isproduced daily.

From the ratio of dispersion to quantity of glass produced it emergesthat for the same specific gather (normalized with respect to thesurface area of the floor), the power expended per unit of productdecreases with an increase in the surface area of the floor.

As can be seen from FIG. 5, the specific consumption also decreases withan increase in the specific gather. In the example of calculation,values of a specific gather ranging from 3250 a 3750 kg/(day m²_(floor)) have been used.

It can be seen how, with a suitable floor surface area and with theabovementioned specific gather values, it is possible to achieve easilyspecific consumption levels of up to 0.6 kWh/kg.

In operating conditions, the quantity of electrical power which flowswithin the molten bath 3 depends on the electrical resistivity of theglass, which varies depending on the chemical composition of the glassitself. The consumption of current depends, not only on the differencein potential at the terminals of the immersed electrodes and theelectrical resistivity of the molten glass bath, but also in decisivemanner on the geometrical distribution of the electrodes.

The confining effect must also be taken into account during thecalculation of the electrical resistance between the immersedelectrodes. In fact, the volume occupied by the molten glass bath hasbeen reduced compared to a conventional electric furnace. Therefore, theinterfaces which delimit the molten bath 3 modify significantly thepotential range, and the simplified theory of infinite means, which isusually adopted in large electric furnaces, is no longer valid.

The potential range also depends on the type of electrical power supplyused: a single-phase alternating voltage system may be taken intoconsideration only in small-size furnaces, while the three-phase systemis generally preferable and obligatory in large-size furnaces.

A homogeneous distribution of the power in the molten bath is essentialfor correct operation of an electric furnace. A second condition, whichis of an operational/design nature, relates to the limit values of thecurrent density in the glass at the electrodes 9.

In the case of industrial glass it is advisable not to exceed a currentdensity of 2 A/cm2, while in the case of glass which is of a highquality or particularly rich in substances which are corrosive for theelectrodes 9, it is advisable not to exceed the density of 0.7 A/cm2.This second condition results in the need to design the electrodes 9with a considerable length and, because of the low head B of the glass,said electrodes must be inserted laterally into the side wall 5 and mustrest on the floor 4 along the whole of their length.

With reference to FIG. 2, Table 1 shows an example of calculation of theapplied voltage Vapp, the current I and the currently density i at theelectrodes 9 (effective values) as a function of the mutual distancesD,D′ between the electrodes 9.

The calculation refers to a furnace with floor 4 having dimensions ofabout 4 m², with a square plan design, the power of which has beencalculated as being approximately 343 kW at the specific gather of 3540kg/(day m2_(floor)). TABLE 1 Characteristic voltage and current data(effective values) of the furnace according to FIG. 2 with single-phasepower supply (ρ_(glass)˜3.45 Ω cm). The condition where there is uniformdistribution of the currents in the molten glass is shown in bold. I₁ =I₄ i₁ = i₄ I₂ = I₃ i₂ = i₃ I_(D) I_(D′) D [cm] D′ [cm] I_(tot) [A] V [V][A] [A/cm²] [A] [A/cm²] [A] [A] 67.5 60.0 4525 76 1454 0.45 3071 0.961454 1617 65.0 65.0 4520 76 1497 0.47 3023 0.94 1497 1526 64.4 66.2 452076 1507 0.47 3014 0.94 1507 1507 62.5 70.0 4524 76 1539 0.48 2984 0.931539 1445 60.0 75.0 4535 76 1582 0.49 2953 0.92 1582 1371 55.0 85.0 457775 1668 0.52 2909 0.91 1668 1242 50.0 95.0 4645 74 1758 0.55 2888 0.901758 1130 45.0 105.0  4741 72 1855 0.58 2886 0.90 1855 1031I_(n) current in the electrode ni_(n) current density at the glass-electrode interface nI_(D(D′)) current in the glass between lateral and central electrode(D′, between central electrodes) of the melting tank according to FIG. 2

In the case where the furnace is powered with a three-phase alternatingvoltage R-S-T, the current density at the electrodes 9 decreases. Thefollowing Table 2 shows the same calculations illustrated in Table 1. Inthis case, with reference to FIG. 2, the two external electrodes areconnected to the phase R, the second electrode from the left isconnected to the phase S and the remaining electrode to the phase T,resulting in a triangular connection which is powered symmetrically.TABLE 2 Characteristic current data (effective values) of the furnaceaccording to FIG. 2 with three-phase power supply (ρ_(glass)˜3.45 Ω cm).The condition where there is uniform distribution of the currents in themolten glass is shown in bold. I₁ = I₄ J_(ST) I₂ = I₃ I_(R) i₁ = i₄ i₂ D[cm] D′ [cm] [A] [A] [A] [A] [A/cm²] [A/cm²] I_(D) [A] I_(D′) [A] 67.2760.45 1462 1605 2779 2779 0.464 0.883 1462 1953 65.00 65.00 1499 15252742 2842 0.476 0.871 1499 1867 62.50 70.00 1540 1445 2709 2909 0.4890.861 1540 1781 60.00 75.00 1580 1373 2683 2976 0.502 0.852 1580 170157.50 80.00 1621 1307 2663 3043 0.515 0.846 1621 1627 57.37 80.26 16231304 2662 3047 0.516 0.846 1623 1623 55.00 85.00 1663 1247 2649 31110.528 0.842 1663 1557 52.50 90.00 1705 1190 2640 3180 0.542 0.839 17051492I_(n) current in the electrode ni_(n) current density at the glass-electrode interface nI_(R (S,T)) phase current R (S, T); in the case of the centralelectrodes the current in the electrode is equal to the phase current Sand TJ_(ST) line current between the phases S and TI_(D(D′)) current in the glass between lateral electrode and centralelectrode (D′, between central electrodes) of the melting tank accordingto FIG. 2

From that described above, it is clear that, with the method accordingto the invention and its implementation by means of an electric furnacein accordance with the claims, it is possible to achieve the predefinedobjects and in particular perform in a cost-effective manner the meltingof vitrifiable materials, in particular for the manufacture of vitreousmosaic materials and ceramic frits as well as for the vitrification ofwaste, using electric power.

In particular, with the method according to the invention it is possibleto provide a cold-crown furnace which is able to lower the temperatureand the quantity of polluting substances contained in the fumesdischarged into the atmosphere, limiting the specific power consumption.Moreover, with the invention it is possible to reduce the time requiredfor changing the vitrifiable material.

The method and the furnace according to the invention are subject tonumerous modifications and variations all falling within the inventiveidea expressed in the claims. All the details may be replaced by othertechnically equivalent elements, and the materials may vary according torequirements without departing from the scope of the invention.

Even though the object of the invention has been described withparticular reference to the accompanying figures, the reference numbersused in the description and in the claims are used in order tofacilitate understanding of the invention and do not limit in anyway thescope of protection claimed.

1. A method for melting vitrifiable materials, in particular for the production of vitreous mosaic materials and ceramic frits as well as for the vitrification of waste, where the primary material must be frequently changed, comprising the following steps: providing a melting tank having a floor and side walls made of refractory material for containing a molten bath, with a predetermined head and at least one channel for discharging the molten materials; introducing a primary batch of vitrifiable materials into said tank via an entry mouth thereof; providing, inside said tank, a plurality of electrodes having a predetermined shape and length, said electrodes having a substantially constant cross-section over their entire length and being so positioned as to melt completely said vitrifiable materials by means of diffused electric currents; depositing a covering layer of vitrifiable materials in the solid state onto the upper surface of said molten batch so as to contain the dispersion of heat from the bath and screen the crown of the furnace; wherein said electrodes are positioned so as to rest at the same level on said floor over their entire length to reduce to a minimum the head of the molten bath, with a consequent reduction in the time required to change the primary batch and the power consumption.
 2. The method according to claim 1, wherein the volume of the primary batch is limited by containing said head within predetermined values depending on the diameter of the electrodes.
 3. The method according to claim 2, wherein said head is kept within values which are between twice and six times the average diameter of the electrodes, with said average diameter being between 1″ and 2″.
 4. The method according to claim 3, wherein the floor surface area of the melting tank and the average specific gather of vitrifiable materials are so selected that the power consumption is kept less than or equal to 0.6 kWh for each kilogram of glass produced.
 5. An electric furnace comprising: a melting tank for containing a molten bath with a floor, side walls channels for discharging the molten materials; means for introducing into said tank a primary batch of vitrifiable materials and for depositing a covering layer on the molten bath having a predetermined head; a plurality of electrodes situated inside said tank so as to melt and keep in the molten state said vitrifiable materials by means of diffused electric currents, said electrodes having an overall length and a substantially constant cross-section over said length and a predetermined position; a crown situated above said floor, all said electrodes being so positioned inside the tank to substantially rest at the same level on said floor so as to reduce to a minimum the head of the molten bath, with a consequent reduction in the time required to change the primary batch and the power consumption.
 6. The furnace according to claim 5, wherein said electrodes are substantially cylindrical and straight and are arranged substantially parallel to each other.
 7. The furnace according to claim 6, wherein said electrodes have one longitudinal and rigidly secured to a side wall of the tank and the other longitudinal end in contact with the opposite side wall so as to be slightly compressed or tensioned at the tip.
 8. The furnace according to claim 7, wherein the distance between said electrodes is selected so as to optimize the distribution of the electric current inside the molten bath.
 9. The furnace according to claim 5, wherein the side wall of said tank has a minimum height which is greater than the maximum value of the head plus the maximum thickness of said covering layers.
 10. The furnace according to claim 9, wherein said minimum height of the side walls of the tank is between 35 and 60 cm with the diameter of said electrodes between 1″ and 2½″.
 11. The furnace according to claim 10, wherein said minimum height is between 40 and 60 cm with the diameter of said electrodes between 1″ and 2½″.
 12. The furnace according to claim 8, wherein said discharge channels extend in said floor at least partially underneath the level of said electrodes to prevent these latter from hindering the flowing out of the molten bathe.
 13. The furnace according to claim 12, wherein said discharge channels comprise at least one main receiving canal connected to the outside of the furnace by means of a discharge gully.
 14. The furnace according to claim 13, wherein said discharge channels comprise a plurality of secondary receiving canals connected to said main canals.
 15. The furnace according to claim 13, wherein said main and secondary canals are transverse to each other and extend completely underneath said electrodes. 